1. Field of the Invention
The present invention relates to an active filter device constituted by a PWM (Pulse Width Modulation) inverter.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of an active filter device in the prior art disclosed, for example, in "Study report of the institutes of electrical and information engineers of Japan in 1984" (Outdoor self-cooling voltage type active filter) published in Apr. 1984.
In FIG. 1, numerals 1a to 1c designate a three-phase AC power source 1, numerals 2a to 2c designate reactors, numerals 3a to 3f designate transistor switches, numeral 4 designates a capacitor, and numeral 5 designates an active filter which is constituted by the transistor switches 3a to 3f and the capacitor 4. The active filter 5 is connected through the reactors 2a to 2c to the three-phase AC power source 1.
Numerals 6a to 6c designate current transformers for detecting input currents of the active filter 5, numeral 7 designates a harmonic current generating load, and numerals 8a to 8c designate current transformers which detect currents flowing in the harmonic current generating load 7.
Numerals 9a to 9c designate fundamental wave removing circuits, numerals 10a to 10c designate arithmetic circuits, and numerals 11a to 11c designate amplifier circuits which amplify outputs of the arithmetic circuits 10a to 10c in multiples of K.
Numerals 12a to 12c designate arithmetic circuits, and numeral 13 designates a PWM wave generating circuit. The current transformers 6a to 6c for detecting input currents, the harmonic current generating load 7, the current transformers 8a to 8c, the fundamental wave removing circuits 9a to 9c, the arithmetic circuits 10a to 10c, the amplifier circuits 11a to 11c, the arithmetic circuits 12a to 12c and the PWM wave generating circuit 13 constitute a control circuit. Each output of the PWM wave generating circuit 13 is outputted to the bases of the transistor switches 3a to 3f, and turns the transistor switches 3a to 3f on or off.
Next, operation will be described.
Currents I.sub.L a to I.sub.L c flowing in the harmonic current generating load 7 are detected as currents I.sub.L a* to I.sub.L c* through the current transformers 8a to 8c, and are inputted to the fundamental wave removing circuits 9a to 9c.
The fundamental wave removing circuits 9a to 9c are composed of high-pass filters, and remove the fundamental component contained in the currents I.sub.L a* to I.sub.L c* and extract harmonic currents I.sub.R a* to I.sub.R c* only. The harmonic currents I.sub.R a* to I.sub.R c* act as current reference signals to the active filter 5, and active filter output currents Ica* to Icc* detected through the input current transformers 6a to 6c are outputted as feedback currents and subtracting components of the arithmetic circuits 10a to which 10c, and adding operation to the harmonic currents I.sub.R a* to I.sub.R c* are applied as adding components; thereby control deviations .DELTA.Ia* to .DELTA.Ic* are obtained.
The control deviations .DELTA.Ia* to .DELTA.Ic* are multiplied by K in the amplifier circuits 11a to 11c at the next stage and outputted as adding components of the arithmetic circuits 12a to 12c at the next stage, and addition to the power source voltages Ea to Ec is executed and the summed output is outputted as control signals Ma to Mc to the PWM wave generating circuit 13.
Thereafter in the PWM wave generating circuit 13, the control signals Ma to Mc are compared with triangular carrier signals (not shown), and command for ON/OFF operation of the base of the transistor switches 3a to 3f are outputted. Corresponding to this, the transistor switches 3a to 3f charge the capacitor 4 and the charged voltage E.sub.1 is subjected to PWM modulation, thereby inverter output voltages E.sub.F a to E.sub.F c are generated and compensation currents Ica to Icc flow at the power source side.
Consequently, if the gain K of the amplifier circuits 11a to 11c is made sufficiently large, the compensation currents Ica to Icc and the harmonic currents I.sub.R a* to I.sub.R c* become nearly equal, and the harmonic currents flowing in the harmonic current generating load 7 can be compensated by the compensation currents Ica to Icc.
Since the active filter device in the prior art is constituted as above described, on account of overhead time by upper and lower arm short circuit preventing dead time of the PWM wave generating circuit 13, mainly, overhead time by the ON-time and the OFF-time of the transistor switches 3a to 3f, and time delay by the offset of circuits such as the amplifier circuits 11a to 11c or the arithmetic circuits 12a to 12c, the active filter output current feedback signals Ica* to Icc* are delayed with respect to the harmonic current reference signals .DELTA.I.sub.R a* to .DELTA.I.sub.R c*; thereby the harmonic compensation capacity of the active filter device, particularly the compensation capacity of higher harmonic current, may be reduced.